This invention relates to a novel compound useful to make an optically anisotropic medium such as a wave plate and an optical low pass filter, a polymerizable liquid crystal composition comprising the compound, and an optically anisotropic medium obtained by polymerizing the composition.
Optically anisotropic media having a fixed oriented structure can be produced by subjecting a polymerizable liquid crystal composition comprising a compound having a polymerizable functional group to orientation processing in its liquid crystal state and then polymerizing the oriented composition, while maintaining in its oriented state, by irradiation with active energy rays such as ultraviolet (UV) rays or electron beams. The optically anisotropic media thus produced show anisotropy in physical properties, such as refractive index, dielectric constant, magnetic susceptibility, elastic modulus, and thermal expansion coefficient, and are useful as a wave plate, a polarizer, a polarizing prism, a waveguide, etc.
JP-A-8-3111 (corresponding to U.S. Pat. No. 5,863,457) (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a polymerizable liquid crystal composition for these applications. The composition disclosed therein has a feature that it exhibits liquid crystalline properties at room temperature but has a drawback of insufficient transparency after polymerization.
JP-A-9-40585 (corresponding to U.S. Pat. No. 5,800,733) teaches that compounds having a folding molecular structure having long chains bonded to the 1- and 2-positions of a benzene ring, respectively, are effective in developing liquid crystalline properties at low temperature, and discloses, as an example, a compound having a xe2x80x94CH2CH2COOxe2x80x94 bond represented by formula (I-d): 
wherein s represents an integer of 3 to 12; R34 represents a fluorine atom, a chlorine atom, a cyano group or an alkoxycarbonyl group; and X1, X2 and X3 each represent a hydrogen atom.
Because of the two long chains introduced for facilitating development of liquid crystalline properties, these compounds have a high molecular weight and a very high viscosity. As a result, in case orientation defects such as disclination occur in orientation, it needs a lot of time to eliminate the defects, resulting in poor productivity.
Accordingly, an object of the present invention is to provide an optically anisotropic medium having excellent transparency with a small haze.
Another object of the invention is to provide a polymerizable compound useful for the production of the optically anisotropic medium.
A still other object of the invention is to provide a polymerizable liquid crystal composition comprising the polymerizable compound.
Other objects and effects of the invention will become apparent from the following description.
The above-described objects of the invention has been achieved by providing a novel polymerizable compound of a linear molecule in which three phenylene groups are linked via a 1,4-bond with one of the two linking groups that link the 1,4-phenylene groups being xe2x80x94CH2CH2COOxe2x80x94 or xe2x80x94CH2CH2OCOxe2x80x94. The inventors found that the compound has a low under limit temperature of liquid crystalline phase owing to this structure and that a polymerizable liquid crystal composition containing the compound has a low under limit temperature of liquid crystalline phase as well. The inventors also found that an optically anisotropic medium obtained by subjecting the composition to orientation processing in its crystal state and then polymerizing the resulting composition with active energy ray irradiation while maintaining the oriented state has high transparency with a small haze.
That is, the present invention provides a polymerizable compound represented by formula (I): 
wherein Q1 and Q2 each independently represent a hydrogen atom or a methyl group; p and q each independently represent an integer of 2 to 18; X1 and X2 each independently represent a single bond, xe2x80x94Oxe2x80x94, xe2x80x94COOxe2x80x94 or xe2x80x94OCOxe2x80x94; L1 represents xe2x80x94COOxe2x80x94 or xe2x80x94OCOxe2x80x94; L2 represents xe2x80x94CH2CH2COOxe2x80x94 or xe2x80x94CH2CH2OCOxe2x80x94; and Y1, Y2 and Y3 each independently represent a hydrogen atom, an alkyl group having 1 to 7 carbon atoms, an alkoxy group having 1 to 7 carbon atoms, an alkanoyl group having 2 to 7 carbon atoms, a cyano group or a halogen atom.
In addition, the present invention also provides a polymerizable liquid crystal composition for producing an optically anisotropic medium, which comprises:
a polymerizable compound represented by the above formula (I); and
a compound represented by formula (II): 
wherein Q3 represents a hydrogen atom or a methyl group; X3 represents a single bond or xe2x80x94Oxe2x80x94; X4 represents a single bond, xe2x80x94Oxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94COOxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94COOxe2x80x94; B1, B2 and B3 each independently represent a 1,4-phenylene group, a 1,4-phenylene group in which one CH group or not adjoining two CH groups thereof are displaced with a nitrogen atom, a 1,4-cyclohexylene group, a 1,4-cyclohexylene group in which one CH2 group or not adjoining two CH2 groups thereof are displaced with an oxygen atom or a sulfur atom, a 1,4-cyclohexenylene group, or a 1,4-cyclohexenylene group in which one CH2 group or not adjoining two CH2 groups thereof are displaced with an oxygen atom or a sulfur atom, and B1, B2 and B3 each may have one or more substituents selected from an alkyl group having 1 to 7 carbon atoms, an alkoxy group having 1 to 7 carbon atoms, an alkanoyl group having 2 to 7 carbon atoms, a cyano group, and a halogen atom; L3 and L4 each independently represent a single bond, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94CFxe2x95x90CFxe2x80x94, xe2x80x94(CH2)4xe2x80x94, xe2x80x94CH2CH2CH2Oxe2x80x94, xe2x80x94OCH2CH2CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94COOxe2x80x94 or xe2x80x94OCOxe2x80x94CHxe2x95x90CHxe2x80x94; Z1 represents a hydrogen atom, a halogen atom, a cyano group, an alkyl group having 1 to 20 carbon atoms or an alkenyl group having 2 to 20 carbon atoms, wherein one or more CH2 groups of the alkyl group or the alkenyl group may be displaced with xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94 or xe2x80x94OCOOxe2x80x94, provided that the displacement does not make an Oxe2x80x94O bond; r represents 0 or an integer of 1 to 18; and s represents 0 or 1.
Since the polymerizable liquid crystal composition contains the compound of formula (I), the under limit temperature of liquid crystalline phase thereof is low. Therefore, it is not necessary to excessively raise a temperature upon producing an optically anisotropic medium by orienting the polymerizable liquid crystal composition in its liquid crystal state and then polymerizing the same while maintaining the oriented state. This affords convenience in handling and suppresses an increase in haze, caused by thermal polymerization or oxidation due to heating, in the resulting optically anisotropic medium. Hence, an optically anisotropic medium having high transparency can be produced from the polymerizable liquid crystal composition of the invention.
In formula (I), p and q are each preferably 3 to 12, particularly preferably 3 to 8. X1 and X2 are each preferably xe2x80x94Oxe2x80x94. Y1, Y2 and Y3 are each preferably a hydrogen atom, a fluorine atom, an alkyl group having 1 to 3 carbon atoms or an alkoxy group having 1 to 3 carbon atoms. Of the alkyl and alkoxy groups preferred are a methyl group and a methoxy group. A hydrogen atom and a fluorine atom are particularly preferred as Y1, Y2 and Y3.
The compounds represented by formula (I) include those represented by formulae (1) through (12) shown blow, in which p and q each independently represent an integer of 2 to 18: 
Of the above compounds preferred are those of formula (1) in which p and q are each independently an integer of 3 to 8.
An example of the synthesis method of the compound according to the present invention is shown below. 
In the above formulae, p, q, Q1 and Q2 are as defined in formula (I).
The compound of the invention is structurally characterized in that rings are linked via a group represented by xe2x80x94CH2CH2COOxe2x80x94 or xe2x80x94CH2CH2OCOxe2x80x94. Synthesis of the chemical structures other than this linking part can be performed by known processes established in the art of liquid crystal compounds in combination with the above-described processes.
The polymerizable liquid crystal composition according to the present invention comprises the compound of formula (I) and the compound of formula (II).
Of the compounds of formula (I) for use in the polymerizable liquid crystal composition, those represented by the following formula are preferred: 
wherein p and q each independently represent 3 to 8.
The polymerizable liquid crystal composition preferably contains the compound of formula (I) in a proportion of 30% by weight or more, more preferably 50% by weight or more, particularly preferably 70% by weight or more.
Of the compounds of formula (II), preferred are those in which X3 is a single bond, and r is 0, i.e., compounds represented by formula (II-1): 
(wherein Q3, X4, B1, B2, B3, L3, L4, Z1 and s are as defined in formula (II), X3 represents a single bond, and r represents 0); and those in which X3 is xe2x80x94Oxe2x80x94, and r is an integer of 1 to 18, i.e., compounds represented by formula (II-2): 
wherein Q3, X4, B1, B2, B3, L3, L4, Z1 and s are as defined in formula (I), X3 represents xe2x80x94Oxe2x80x94, and r represents an integer of 1 to 18.
In formula (II-2), r is preferably 2 to 12, more preferably 2 to 8, particularly preferably 2 to 6. Of the compounds represented by formula (II-1), further preferred are those represented by formulae (III) and (IV): 
wherein Q3 represents a hydrogen atom or a methyl group; and R1 represents an alkyl group having 1 to 10 carbon atoms; 
wherein Q3 represents a hydrogen atom or a methyl group; and R2 represents an alkyl group having 1 to 10 carbon atoms. The compound of formula (III) and the compound of formula (IV) can be used either singly or in combination. In other words, the polymerizable liquid crystal composition of the present invention preferably comprises the polymerizable compound of formula (1) and at least one of the compound of formula (III) and the compound of formula (IV). Where the compound of formula (III) and the compound of formula (IV) are used in combination, it is preferred that the proportions of these compounds be equal for decreasing the under limit temperature of liquid crystalline phase of the polymerizable liquid crystal composition.
Specific examples of the compound of formula (II-2) include those represented by formulae (a-1) through (a-15) shown below, in which r and Q3 are as defined in formula (II); and R3 represents a hydrocarbon group having 1 to 20 carbon atoms: 
The proportion of the compound of formula (II) in the polymerizable liquid crystal composition is preferably 30 to 70% by weight, more preferably 30 to 50% by weight.
The polymerizable liquid crystal composition can further comprise known polymerizable liquid crystal compositions in addition to the compounds (I) and (II).
The liquid crystalline phase temperature range of the polymerizable liquid crystal composition according to the invention usually is from 20 to 80xc2x0 C. The liquid crystalline phase is preferably a nematic phase which shows satisfactory alignment.
Since the liquid crystal composition of the invention keeps its liquid crystalline phase even in a low temperature, a high degree of orientation can be fixed in low temperatures in which thermal fluctuations are suppressed. As a result, satisfactory uniformity of orientation can be secured, which is highly advantageous for making optically anisotropic media having excellent transparency with a small haze. From this viewpoint, it is preferred that the composition of the invention be designed to have an under limit temperature of liquid crystalline phase of 40xc2x0 C. or lower, especially around room temperature (i.e., 25xc2x0 C.).
Next, the method for orienting the polymerizable liquid crystal composition of the invention is described below.
The polymerizable liquid crystal composition can be oriented by applying the composition to (1) a substrate having been rubbed with cloth, etc., (2) a substrate having an organic thin film formed thereon which has been rubbed with cloth, etc. or (3) a substrate having an SiO2 layer formed by oblique vacuum deposition, or by filling the composition into a gap between a pair of these substrates.
The composition can be applied to a substrate by spin coating, die coating, extrusion coating, roll coating, wire bar coating, gravure coating, spray coating, dip coating, printing or a like coating method.
The orientation of the polymerizable liquid crystal composition can also be achieved by other methods used for liquid crystal materials such as flow-induced orientation, electric field orientation or magnetic field orientation. These means can be used either singly or as a combination of two of more thereof. Photo-induced orientation can be used instead of the rubbing technique.
A polyimide film, which is used in usual twisted nematic (TN) or supertwisted nematic (STN) devices, can be used to align the polymerizable liquid crystal composition at a tilt angle with respect to the substrate. Orientation processing using a polyimide film is preferred because the molecular orientation structures inside an optically anisotropic medium can be further precisely controlled. Where the alignment is controlled in an electric field, it is preferred to use a substrate having an electrode layer. A substrate having an organic thin film on the electrode layer is particularly preferred. The organic thin film is preferably a polyimide thin film.
The direction of orientation with respect to the substrate is of choice according to the intended use or desired functions of the article to be made.
A photopolymerization initiator is usually added to the polymerizable liquid crystal composition of the invention to improve the polymerization reactivity.
The polymerizable liquid crystal composition is polymerized by irradiation with active energy rays for securing rapid progress of polymerization. UV light is preferably used as active energy rays. Either a polarized light source or a non-polarized light source can be used. The intensity of UV rays is preferably 0.1 mW to 2 W/cm2. With an intensity lower than 0.1 mW/cm2, a very long time will be needed to complete photopolymerization, resulting in poor productivity. Light intensities higher than 2 W/cm2 tend to deteriorate the liquid crystal composition.
The temperature at the time of the irradiation is preferably within a range in which the liquid crystal composition retains its liquid crystal state. The temperature is preferably as close as possible to room temperature, typically at 25xc2x0 C., for avoiding induction of unintended thermal polymerization.
Next, the optically anisotropic medium of the present invention is described below.
The optically anisotropic media according to the present invention can be obtained by orienting the polymerizable liquid crystal composition in its liquid crystal state by the above-described methods and irradiating the composition with the above-described active energy rays to cause the composition to polymerize while retaining its oriented state.
An optically anisotropic medium that is used to widen the viewing angle of a liquid crystal display is obtained by orienting the molecules in the direction making 10 to 80xc2x0, preferably 20 to 70xc2x0, with the substrate or by making hybrid alignment.
An optically anisotropic medium that is used as a polarizer or an optical low pass filter is obtained by orienting the molecules in the direction making 30 to 60xc2x0, preferably 40 to 50xc2x0, still preferably 45xc2x0, with the substrate or by hybrid alignment.
In applications as a polarizer or an optical low pass filter, the film is often made thicker (50 xcexcm or greater) than for use as a wave plate. The material being the same, a thicker film involves larger light scattering. Therefore, an optically anisotropic medium for making a practical optical device is required to cause little light scattering, i.e., to have a small haze. The polymerizable liquid crystal composition of the invention can provide an optically anisotropic medium having a reduced haze of 3% or less. In applications as a compensator to be integrated into a liquid crystal cell of a reflection type liquid crystal device, it is important to reduce a haze to increase transparency. This is particularly important for use as a quarter wave plate.
The optically anisotropic medium thus prepared can be used either as formed on the substrate or as stripped off the substrate used for alignment control. A plurality of the resulting optically anisotropic media can be used as stacked, or the medium can be adhered to another substrate.
The present invention will now be illustrated in greater detail with reference to the following Examples, but the invention should not be construed as being limited thereto. All the xe2x80x9cpartsxe2x80x9d are given by weight unless otherwise indicated.